Engines may be configured with an exhaust heat recovery system for recovering heat from exhaust gas. The exhaust heat may be utilized for heating the engine coolant which may provide heat to the engine and also warm the passenger cabin, thereby improving engine and fuel efficiency. Further, exhaust gas may be recirculated to the intake manifold via an EGR system and used to reduce exhaust NOx emissions. An exhaust gas recirculation (EGR) cooler may be coupled to the EGR system to bring down the temperature of recirculated exhaust gas before it is delivered to the intake manifold. A combined heat exchange system may be used for both exhaust heat recovery and EGR cooling. A diagnostic procedure may need to be periodically or opportunistically carried out to monitor the different components of the engine system.
Various approaches are provided for diagnostics of an exhaust heat exchanger and an EGR cooler. In one example, as shown in U.S. Pat. No. 6,848,434, Li et al. discloses a method for diagnosing an EGR cooler coupled to an EGR delivery passage. An effectiveness factor for the EGR cooler is computed based on factors including exhaust gas temperature, EGR cooler outlet temperature, and engine coolant temperature. The effectiveness factor is compared to a threshold, and if the effectiveness factor is lower than the threshold, malfunction of the EGR cooler is determined. The threshold may be based on EGR flowrate through the delivery passage.
However, the inventors herein have recognized potential issues with the above approach. As one example, in embodiments having a combined heat exchange system where a common heat exchanger is used for both exhaust heat recovery and EGR cooling, it may not be possible to reliably detect degradation of the EGR cooler function using a single effectiveness factor. In particular, it may be difficult to differentiate issues with the EGR cooling functionality of the combined heat exchange system from issues with the exhaust heat recovery functionality. Further, due to the presence of a plurality of valves enabling different combinations of EGR and exhaust heat recovery functionality, it may be difficult to parse out issues associated with the valves from issues associated with the heat exchanger or with a coolant line circulating coolant through the heat exchanger.
The inventors herein have identified an approach by which the issues described above may be at least partly addressed. One example method comprises, indicating degradation of a heat exchange system diverting exhaust, via a diverter valve, from downstream of an exhaust catalyst into a heat exchanger in an exhaust bypass, the indicating based on each of a first exhaust temperature and an exhaust pressure estimated upstream of the heat exchanger, a second exhaust temperature estimated downstream of the heat exchanger, and a temperature of coolant circulating through the heat exchanger. In this way, by using a plurality of temperature, pressure, and diverter valve position sensors, each component of the exhaust heat exchange system may be reliably diagnosed.
In one example, an engine system may be configured with a single heat exchanger for EGR cooling and exhaust heat recovery. The heat exchanger may be positioned in an exhaust bypass passage disposed parallel to a main exhaust passage, and a diverter valve coupled to the main exhaust passage may be used to enable exhaust to be diverted into the bypass passage or directed through the main passage into the tailpipe. An EGR delivery passage including an EGR valve may be coupled to the bypass passage downstream of the heat exchanger. One or more temperature and pressure sensors may be coupled to the bypass passage upstream and downstream of the heat exchanger. Also, a temperature sensor may be housed in the outgoing coolant line of a coolant system fluidically coupled to the heat exchanger. The heat exchanger system may be operated in a plurality of modes by adjusting a position of the diverter valve and the EGR valve, and based on the operational mode, inputs from each of the temperature and pressure sensors may be periodically or opportunistically used for diagnosing the components of the heat exchange system. As one example, during an operational mode where only exhaust heat recovery is being carried out, the presence of a higher than threshold pressure upstream of the heat exchanger may be used to infer degradation of the heat exchanger (such as due to clogging). During the same mode, a drop in coolant temperature may be used to infer degradation of the coolant system (such as due to clogging of the coolant line). As another example, during an operational mode where both exhaust heat recovery and EGR delivery is being carried out, a lower than threshold coolant temperature may be used to infer degradation of the heat exchanger acting as an EGR cooler. The thresholds applied during the exhaust heat recovery only mode may be distinct from those applied during the combined exhaust heat recovery and EGR delivery mode. In each mode, inputs from a position sensor coupled to the diverter valve together with inputs from the temperature and pressure sensors may be used to infer degradation of the diverter valve. Further, during an operational mode where exhaust is not routed via the bypass passage, the various temperature and pressure sensors may be diagnosed based on exhaust temperature upstream of the heat exchanger being substantially equal to the exhaust temperature downstream of the heat exchanger and the exhaust pressure being substantially equal to atmospheric pressure.
In this way, by using existing temperature and pressure sensors coupled to a bypass passage of the exhaust system, on-board diagnostics of a plurality of components of a combined heat exchange system may be effectively carried out. By comparing exhaust temperatures measured upstream and downstream of the heat exchanger to temperatures expected at those locations based on the operational mode of the combined heat exchanger system a blockage in the heat exchanger may be reliably detected. By comparing the exhaust temperature changes to coolant temperature changes in each mode, heat exchange issues associated with the heat exchanger may be better differentiated from those associated with the coolant system coupled to the heat exchanger. The technical effect of adjusting the temperature and pressure thresholds applied during the diagnostics based on the mode of operation of the combined heat exchange system (such as based on whether the system is being used for only exhaust heat recovery or for exhaust heat recovery and EGR delivery) is that the different functionalities of the combined heat exchange system can be independently diagnosed using the same set of sensors. By opportunistically monitoring the health of each of the sensors used for the diagnostic process, reliability of the on-board diagnosis of the heat exchange system may be improved. By enabling diagnostics of the exhaust heat exchange system to be carried out reliably and accurately, the fuel economy and emissions benefits of a combined heat exchange system may be extended over a wider range of engine operating conditions.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.